Apologies for the lengthy update — tl;dr — I replaced a lot of parts, most of it didn't make a difference, but tightening up the advancer did. Now I think I have a "conventional" carb tuning problem (I hope). Unfortunately, I still haven't wholly resolved the issue, though I've made some progress.
:: begin the blow-by-blow ::
I replaced the points plate with a new aftermarket assembly (I know that Diachi gets a lot of hate—I have some Toyo points on order). It retimed nicely, though the Daichi points do seem a bit cockeyed compared to the TEC ones.
To rule out vacuum leaks (though I never found any), I replaced the carb boots, the o-rings on the manifold between the carbs and the jugs, and the rubber boots on the airbox. I also replaced the float bowl gaskets—which were a royal pain to get back in—with nicely shaped ones from 4-into-1.
While I was at it, I swapped in aftermarket jets — not sure of the brand. They were from 4-into-1s kits. Size #40. The mains are #100.
New ones are at left.
I did this after realizing that despite my initial efforts, carb dips, etc., at least two of the pilot jets were still a bit clogged. (I wish I had made a note of which)
After all of that, I had high hopes, but the issues continued — hanging idle, slow-to-return idle, etc.
Looking more closely at the advancer, I realized that in some cases the weights were not returning to make contact with the cam. I trimmed down the advancer springs 1/2 turn and reshaped the ends (per HondaMan's suggestion and as discussed on a number of threads). Now it closes with a nice, confident *snap*.
Throwing that back in there, I noticed an immediate difference. Much more snappy return to idle. I took her out for a ride, and though she behaved well, as the engine heated up, the hanging idle issue began to reappear — sticking around 2.5 - 3k.
Before I tweaked the advancer, it was hard to find a pattern. Now, I feel like I'm starting to see one.
When the bike doesn't return to idle, I'd say 3 outt've 5 times, blipping the throttle seems to *break* it out of the hover, and it drops down to idle. The other times, it'll actually just go higher, almost as if the additional throttle is "added" to the RPM. When that happens, no amount of blipping the throttle brings it down — though under load, you can bring it back down. That seems like an indunction leak. But I can't find it. I double-checked.
I started playing with a few things — when it's sticking high, I tried slowly moving the choke to mostly on (that is, closed; richer). The RPMs don't change, but you can hear a slight difference.
Then I blipped the throttle, and *boom* it drops down to idle.
To confirm that the idle mixture was overly lean, I took out the airbox, and enriched the mix by taping over the inlet. (I first tried it with my hand). When the mixture is enriched like that, it does seem to reliably return to idle.
ANYHOW — my operating theory, now, is that I have a lean pilot jet. That could be because the aftermark jets are no good (I'm thinking of recleaning my Keihin's and putting them back in), or some other reason? I've checked the float levels many times, but perhaps those are the issue? I pulled the plugs, but they were all sooty from the enrichened tests, that I think I'll have to do it again with new ones.
At any rate, I do feel optimistic having (hopefully) graduated the issue from a confusing head-scratcher (for me at least), to what seems to be a more normal carb tuning problem. So what's next?
- Try different pilot jets
- Double-check everything is clean/clear in the carbs
Many thanks for all of the advice. It's been a bit of a slog, but I feel like I'm learning a bit about the dark arts of carburetors, which is fun...would be nice to get everything 100% (or at least, 90%) as riding weather starts rolling in properly...
